32 
23. 
41. 
EMBRYOLOGICAL MONOGRAPHS. 
The digestive cavity itself shows the first trace of differentiation into intestine, stomach, and cesoph- 
agus. 
2. Profile view of an embryo, somewhat older than fig. 21, at the beginning of the fifth day ; the intestine, ec, 
stomach, d, and cesophagus, are well separated ; the anal vibratile cord (v/’) bulges out considerably 
beyond the depression in the ventral side of the Pluteus. 
Embryo at the end of the fifth day, seen from the mouth side ; the water-tubes (w, w!) are only slightly con- 
nected with the digestive cavity ; they also show a difference in size. The original limestone rod has given 
off a shoot, the rod of a new arm to be developed at v. - 
. Fig. 23 seen from the anal extremity, to show the great change of form which has taken place from the early 
cylindrical shape of the embryo. 
. A profile view of fig. 23. The embryo has become pear-shaped, the esophagus has bent over to reach the 
ventral side ; the anal opening is also somewhat ventrally placed ; the depression at m, where the new 
mouth is to be formed, is in contact with the cesophagus ; the anal and oral vibratile cords have increased 
in prominence. 
. A profile view of an embryo, taken at the beginning of the seventh day. The mouth, m, is open ; the water- 
tube w/! reaches nearly to the dorsal surface. The currents, which previous to this stage had carried the 
food through the only opening, a, into the digestive cavity as far as 0, and then were reversed to eject the 
digested matter, now come in through the mouth, m, pass through the cesophagus, 0, rotate about in the 
stomach, d, and pass out through the first-formed opening, the anus, a, which is hereafter only used to 
eject the food. 
. A Pluteus at the end of the eighth day, seen obliquely from the ventral side, to show the course and shape of 
the vibratile cord. 
. Profile view (actinal) of a stage slightly older than that of fig. 27. 
. The same as fig. 28, seen from the ventral side. 
. Somewhat more advanced Pluteus, to show the changes the vibratile cord has undergone since the stage of 
fig. 27 ; seen obliquely from the ventral side. 
A Pluteus during the tenth day, seen in profile, shows the beginning of the small arms e!” and erv. 
2. The same as fig. 31, seen from the mouth side. The arms e! have been greatly developed ; the differentiation 
of the intestine, c, the stomach, d, and the esophagus, is quite complete. First appearance of the vibratile 
epaulettes, v!’. The water-tubes have not yet united, and have not greatly increased in size from the pre- 
ceding stage. 
3, Profile of a Pluteus during the twenty-third day. The arm e!/ has increased greatly in length, and a con- 
siderable increase of the vibratile epaulettes is to be noticed. 
. The same as fig. 33, seen from the dorsal side, to show the relation of the rods of the arms e¢’, e!!, e!!’, to 
each other ; the water-tube w! shows the first indication of a large tentacular lobe. 
. Fig. 34, seen somewhat obliquely, in an attitude similar to that of figs. 27 and 30, to show the connection of 
the different parts of the vibratile cord. 
. A much more advanced Pluteus, fished up from the surface. The rods extending into the arms are made up of 
three sets of rods united by short transverse bars ; the whole oral extremity of the larval body has greatly 
lengthened; the arms ¢!!! and erv are longer than in the preceding stages, e/, e!’, and e/!/ being nearly of 
equal length ; the arms show a tendency to a paired arrangement of e!, e!’, and e!’, etv. Additional ten- 
tacular lobes have been formed in the water-tube w!, and the water-tubes have become united in the oral 
extremity beyond the saclike pouch of the mouth of the Pluteus. 
. Fully developed Pluteus of Strongylocentrotus, in which the young Sea-urchin has already encroached some- 
what on the anal extremity ; its spines are quite well marked ; the vibratile epaulettes have acquired a 
great size; two very prominent spots, s’, s’, in the arms e¢/, e!’, At the base of the oral extremity of the 
mouth pouch a rudimentary appendage, f, appears; this is perhaps the homologue of the brachiolarian 
appendages of the Brachiolaria of Asteracanthion. The arms ¢/, e/’, and e!!/, eIV, are now of nearly equal 
length, and arranged in pairs. 
8. Fig. 37 seen from the oral extremity of the Pluteus. 
. Profile view of fig. 37. 
. A young Echinus, immediately after the resorption of the Pluteus, seen from the abactinal side. The anal 
opening cannot be traced in the youngest specimens, though it is very apparent in somewhat more advanced 
stages (a, tig. 43). 
A young Echinus, somewhat more advanced than the stage of fig. 40, seen from the actinal side ; the tenta- 
cles have become more slender; the odd tentacle ¢! especially, when fully extended, more than equals the 
diameter of the test ; the interambulacral spines of this side are nearly as long as the diameter of the test. 
The actinal system is not well separated from the coronal test. The two tentacles nearest the actino- 
stome are remarkable for the great development of the sucking disk. 
. The same as fig. 41, seen from the abactinal side ; the spines of the abactinal area are remarkable for their 
fan-shaped spiny extremities. 
